Setup of Xeams after GFIME

I need to setup Xeams for my DKIM requirement and need to place after GFI for outbound traffic: Exchange --> GFI --> Xeams --> Firewall --> Internet How to configure GFI MailEssential to send mails through Xeams server, which will apply the DKIM signature to the emails?

Contacted the support, replied that they cant help as they are not trained to how to setup the DKIM signing server, also they cannot guide me on how to configure the outbound email traffic to another server passing from GFI-ME. my current mail flow is: Exchange --> GFI --> Firewall --> Internet And I just need to send email from GFI to Xeams server to add the DKIM signature to the out going emails.